SNMP Protocol Overview, History and General Concepts
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I have described the TCP/IP Internet Standard Management Framework (SNMP Framework) as being information-oriented. A specific decision was made in the design of the SNMP Framework to decouple the management information conveyed between SNMP agents and SNMP managers from the protocol used to carry that information. This provides numerous benefits to the technology as a whole, chief among them flexibility and modularity.
In this model, the operation of the management protocol is not defined in terms of specific commands made to check the status of a device or change how it operates. Instead, the protocol is defined in terms of management information variables called objects, and a communication protocol that allows these objects to be either examined or changed by a network administrator. I describe this concept thoroughly in the topic that overviews the SNMP Management Information Base (MIB) and the Structure of Management Information (SMI).
The MIB and SMI spell out the rules for how MIB objects are created and described. These MIB objects describe the types of information that can be read from the device or written to the device. The last piece of the puzzle is the actual protocol that is responsible for these read and write operations. This is the Simple Network Management Protocol itself, which I give the somewhat redundant name SNMP Protocol to differentiate it from the SNMP Framework.
The result of the separation of the protocol from the management information it carries is that the protocol itself becomes significantly reduced in complexity. Instead of the SNMP Protocol having to define dozens or even hundreds of operations that specify particular network management functions, it only has to deal with the transmission of MIB object information between SNMP agents and managers. The SNMP Protocol does not itself pay attention to what is in these objects; it is merely concerned with moving them around. In some ways, the SNMP Protocol is the only really simple part of SNMP!
The history of the SNMP Protocol goes back to the predecessor of the SNMP Framework, the Simple Gateway Monitoring Protocol (SGMP), which was defined in RFC 1028 in 1987. SGMP was designed as an interim solution for network management while larger issues were being explored as I explained in the overview of the SNMP Framework. However, this standard is where many of the basic design concepts underlying the modern SNMP Protocol can be found.
The SGMP standard specified the basic design model used in SNMP, by describing the SGMP protocol in terms of only retrievals of, or alterations to, variables stored on an Internet gateway (router). The standard also outlines the small number of protocol operations that are still the basis for SNMP's operation today.
The first version of the SNMP Framework, SNMPv1, included the first formal definition of the SNMP Protocol, in RFC 1067 (later revised by RFCs 1098 and 1157). This standard refines the protocol operations given in the SGMP document. It makes the operation of the SNMP Protocol fit into the overall SNMP Framework, working with formally-defined MIB objects.
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